Large turbine generators contain cylindrical rotors up to 2 m in diameter which revolve at speeds of up to 3600 rpm. These rotors contain windings requiring electrical insulation to prevent inter-turn short circuits between the electrical conductors. If the electrical insulation becomes too hot, either due to improper operation or blocked cooling channels, the insulation degrades, eventually allowing inter-turn shorts to occur and leading to severe problems (including a rotor failure). Thus, there is a need to continuously measure the temperature of rotor windings, and in particular, the temperature of the hot spots of the winding. Such temperature information will provide an advance warning of when the insulation may fail. Currently the only rotor temperature information available is an average rotor temperature inferred from a resistance measurement. Temperature measurements with conventional methods are difficult due to the harsh environment existing within the rotor combined with a very high rotational speed of the rotor.
An indirect rotor temperature measurement can be implemented using a phosphor-based surface temperature measurement technique disclosed in U.S. Pat. No. 4,560,286. In this method a phosphor mixed with a suitable binder is painted on the surface of the rotor and an optical fiber is used to deliver pulsed ultraviolet light from a stationary source. This source (typically a laser) excites the phosphor to fluoresce in the visible light spectrum. The visible emission is delivered to the detector by a second optical fiber and the temperature of the illuminated surface is determined from an analysis of the decay time of this signal. Using this technique, the rotor surface temperature directly below an optical fiber can be measured, provided there is suitable fiber access, for example, via ventilation channels in the stator.
This method is only suitable for monitoring the surface temperature of the rotor, since the decay in the fluorescence must be visible by the second (receiving) fiber for several tens of microseconds. During this time, the rotor surface will move several centimeters, which the receiving fiber can accommodate in its field of view. However, it is the temperature of the insulation at the bottom of cooling vent slots, FIG. 1, that is of more importance in monitoring rotor winding insulation temperatures. Unfortunately, these vent slots are typically only 0.5 cm across and up to 10 cm deep. If the "standard" fluorescence technique is used, the receiving fiber cannot view the special phosphor paint long enough to determine the rate of light decay, since the rotor is moving.